An Update on the LHC Monojet Excess

TL;DR

This paper updates previous LHC jet+MET search analyses, confirming a persistent monojet excess that suggests potential new physics involving a heavy colored state decaying to a quark and an invisible particle, with a significance of up to 3.3σ.

Contribution

It provides new ATLAS results supporting the monojet excess and proposes analysis strategies to enhance the signal-to-background ratio for future studies.

Findings

Consistent monojet excess observed in new ATLAS data.

Combined data shows a 3.3σ local significance for new physics.

Enhanced analysis cuts can improve signal detection by 1.5 times.

Abstract

In previous work, we identified an anomalous number of events in the LHC jets+MET searches characterized by low jet multiplicity and low-to-moderate transverse energy variables. Here, we update this analysis with results from a new ATLAS search in the monojet channel which also shows a consistent excess. As before, we find that this "monojet excess" is well-described by the resonant production of a heavy colored state decaying to a quark and a massive invisible particle. In the combined ATLAS and CMS data, we now find a local (global) preference of 3.3$\sigma$ (2.5$\sigma$) for the new physics model over the Standard Model-only hypothesis. As the signal regions containing the excess are systematics-limited, we consider additional cuts to enhance the signal-to-background ratio. We show that binning finer in $H_T$ and requiring the jets to be more central can increase $S/B$ by a factor of ${\sim} 1.5$.